The present invention relates to a method for the preparation of carbon fibers with improved surface properties or, more particularly, to a method for the preparation of carbon fibers of which the surface has remarkably improved resistance against oxidation as well as greatly decreased reactivity with molten metals and the like when the carbon fibers are used as a reinforcing material in a metal-based composite material.
As is well known, carbon fibers are manufactured by the infusibilization and carbonization treatment of fibers or filaments of rayon, polyacrylonitrile, lignin, resinous pitch and the like material in an atmosphere of an inert gas and carbon fibers are widely used in recent years in a wide variety of industrial applications or, in particular, as a reinforcing material in a plastic- or metal-based composite material by virtue of their outstandingly high tensile strength and elastic modulus as well as excellent heat resistance. Carbon fibers are, however, of course not free from several problems and disadvantages including the limitation in the resistance against oxidation at high temperatures and low affinity of the surface thereof with plastics and metals as the matrix material of the composite materials. Moreover, carbon fibers may react with molten metals to form a metal carbide so that the use of carbon fibers as a reinforcing material in a metal-based composite material is largely limited. Several methods have of course been proposed to overcome the above described problems of carbon fibers by the surface treatment thereof but none of such methods is practically feasible due to the complicated processes to practice the method.
Turning now to a substitute for carbon fibers free from the disadvantages inherent to carbon fibers, fibers of silicon carbide developed recently constitute a class of promising materials having excellent resistance against oxidation at high temperatures, good wettability or affinity with plastics and metals and inertness to molten metals. A problem in silicon carbide fibers is the extreme expensiveness thereof as a result of the expensiveness of the starting material which is a very specific organosilicon polymer and the complicate process for the manufacturing including the synthesis of the organosilicon polymer, spinning thereof into filaments, infusibilization and calcination.
Therefore, it may be an idea that a high-performance, heat-resistant fibrous material of practical value would be obtained if a convenient and economical method is developed for providing carbon fibers with a cladding layer of silicon carbide although no such a method with practicability is known in the prior art.